The subject matter of the present invention relates generally to gas pollution control apparatus and methods, and in particular, to such pollution control apparatus and method for treating hot exhaust gas using wood members as a filter media to remove pollutants and to conserve heat energy as well as to dispose of the pollutants by using the contaminated filter media in the production of a commercial product. In the pollution control apparatus and method of the present invention the wood members used as the filter media may be wood chips or lumber supposed in a stack within a container to remove pollutants from the hot exhaust gas by depositing solid particles and condensing hydrocarbon vapors of such pollutants on the surface of such wood chips or lumber. The stack of wood chips may be supported in a vertical silo container, while the lumber is supported in stacks within a lumber kiln, where such chips or lumber are dried and also serve as a filter media. The contaminated surface of the lumber is removed by a planer after unloading such lumber from the kiln, to produce contaminated wood chips. The contaminated wood chips from the silo or from the lumber planer are then further dried and processed to produce a chip board or other commercial wood products thereby disposing of the pollutants.
A wood drying system in accordance with the present invention employs a hot air dryer of wood or organic material, such as a rotary wood chip dryer or a wood veneer dryer, whose hot exhaust gas contains pollutants including solid particles and hydrocarbon vapors. This dryer exhaust gas is transmitted to a lumber kiln for drying the lumber thereby conserving heat energy and for removing from such gas solid particle pollutants which are deposited on the rough surface of the lumber. The kiln exhaust gas is then transmitted through a filter stack of wood chips which may be contained within a vertical silo, for removing remaining pollutants from the gas by causing the hydrocarbon vapors to condense on the wood chips and blocking the passage of the solid particles with such wood chips. The contaminated wood chips are discharged from the bottom of the silo then mixed with clean wood chips and conveyed to the rotary dryer for further drying and subsequent use of such dried chips to manufacture chip board or other commercial wood products. The rough sawn surfaces of the lumber catch many solid particles from the dryer exhaust air and the decrease in temperature within the kiln may cause some of the hydrocarbon vapors to condense onto such surface of the lumber. After kiln drying, the lumber is removed from the kiln and the contaminated surface layer of the lumber is removed by a planer to produce smooth finished lumber. The removed surface layer may produce other contaminated wood chips which are also transmitted back to the rotary dryer for drying and subsequent use in the production of chip board.
While any source of hot air can be employed for the rotary wood chip dryer, it is most economical to use a wood dust burner which burns fine wood particles, such as sander dust, but mixing such wood particles with air and causing combustion thereof. The combustion products carried by such hot air contain pollutants including solid particles such as ash and hydrocarbon vapors. Most of the solid particles of ash are removed by a cyclone furnace separator connected to the burner ignition chamber. However, the hot air supplied to the wood chip dryer by such a burner still contains much pollution in the form of solid particles and hydrocarbon vapors which are transmitted through the rotary dryer to dry the wood chips therein. Additional hydrocarbon vapors are emitted from the wood chips in the rotary dryer so that the dryer exhaust air contains much pollution which must be removed by the lumber and/or wood chip filter media in the manner of the present invention.
Previously it has been proposed to filter a stream of hot exhaust air using inorganic filter media such as sand, rocks or porous ceramic filters. Such filters have the disadvantage that they must be cleaned to remove the captured pollutants actually by washing with water before reuse of such filter media. This also results in contaminated waste water containing pollutants which presents a waste disposal problem. In addition, wet scrubbers have been used to filter pollutants from a stream of hot exhaust air, but these consume much electrical power and also have a contaminated waste water disposal problem. Electrostatic precipitators can also be used to filter such exhaust gas but they are extremely expensive and must be cleaned frequently by washing which creates a similar waste water disposal problem. These problems are avoided using the replaceable wood filter media of the present invention which is used to manufacture commercial wood products to dispose of the pollutants removed by such filter media.